Here's my thought on this:
We are "audioholics", we're generally always going to encourage you to go for the best!
However, unless I'm misremembering, you said you were not a bass head, and 4 Hsu (or equal) ported 12" subs (or 4 sealed 15" subs, which is what I have) is a lot of low frequency energy to pump into any residential room. Of course, there is more/better to be had from a no-compromise perspective, but the question of "how much is enough" is one that ultimately only you can answer.
I can tell you that for me, four 15" sealed subs is more than I need! I never work them hard. I've forgotten how big a volume you are filling, but the immediate room my system is in is 4230 cu.ft.. If I add permanently open spaces to that area it gets just over 9000 cu.ft.
I do like the thrill of my subs causing a visceral sensation when a dinosaur stomps and my adrenaline kicks in, and 4 sealed 15" subs delivers that for me! I'm sure if I had 4 ported 15" VTF-3 to A-B compare, the VTF-3's would impress, but I do not feel deprived in the least with what I have (otherwise I would have changed it by now - no WAF concerns, but I do like my room more open, given the choice).
I have a Denon AVR which allows me to setup differently for different sources. So, I have my DVD input set to run the subs 4-5dB hot to get the "WHUMPF" I like.
I actually have my CD input set to -2dB! I think Audyssey did a good job of setting the level, but there are so many songs where the recording engineer recorded the bass hot (or maybe they did not use subs when mixing). For me, this totally destroys music (the attacks at higher frequencies seem to lose resolution - in other words it sounds sloppy instead of tight)! I am not one to adjust for every song that comes on, so I have found -2dB to be the best compromise of still having good bass on a well recorded songs, but not allowing the recordings of great music with heavy bass to totally suck!
(For my music system, I am looking at some options that have a subwoofer volume level on the remote for easy adjustments on the fly.)
OTOH, so much of this has to do with your room and how much room gain you get and how well Audyssey (assuming you are using it or another room correction feature) handles your specifics. Fortunately, the subs you have been considering all have good tuning options so hopefully you can get it good enough for Audyssey to be able to handle the rest (tune your subs as best you can
before Audyssey).
VTF-2:
https://www.audioholics.com/subwoofer-reviews/hsu-vtf-2-mk5/conclusion
ULS-15:
https://www.audioholics.com/subwoofer-reviews/hsu-uls-15-mk2/conclusion.
Good luck with your decision! The good thing is I don't think any of the gear you are considering would disappoint!
